Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide systems and methods for frequency adjustment based on duty cycles. Merely by way of example, some embodiments of the invention have been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 (e.g., a power converter) includes a system controller 102, a primary winding 130, a secondary winding 132, an auxiliary winding 134, a switch 120, a current sensing resistor 166, diodes 108, 110, and 144, capacitors 106, 150, 152, and 154, an electromagnetic interference (EMI) filter 180, a rectifying bridge 182, resistors 104, 153, 181, and 183, and an isolated feedback component 103. The isolated feedback component 103 includes resistors 160, 162, 168, and 186, a capacitor 164, a three-terminal regulator 172, and an opto-coupler 170.
For example, the power switch 120 includes a bipolar junction transistor. In another example, the power switch 120 includes a field effect transistor (e.g., a metal-oxide-semiconductor field effect transistor). In yet another example, the power switch 120 includes an insulated-gate bipolar transistor. As an example, the system controller 102 includes terminals (e.g., pins) 112, 114, 116, 118 and 199. As another example, the system controller 102 is a chip, which includes the pins 112, 114, 116, 118 and 199.
As shown in FIG. 1, an alternate-current input 198 is processed by the EMI filter 180, and the rectifying bridge 182 provides an input voltage 197 for the operations of the power conversion system 100. The power conversion system 100 uses a transformer including the primary winding 130 and the secondary winding 132 to isolate a primary side and a secondary side of the power conversion system 100. Information related to an output voltage 156 on the secondary side can be extracted through a voltage divider including the resistors 186 and 160.
The isolated feedback component 103 generates a feedback signal 158 based on information related to the output voltage 156. The controller 102 receives the feedback signal 158, and generates a drive signal 122 to turn on and off the switch 120 in order to regulate the output voltage 156. If the power switch 120 is closed (e.g., being turned on), the energy is stored in the transformer including the primary winding 130 and the secondary winding 132. The closed power switch 120 allows a current 124 to flow through the primary winding 130. The current 124 is sensed by the resistor 166 and converted into a current sensing signal 126 (e.g., Vcs) through the terminal 114 (e.g., terminal CS). Then, if the power switch 120 is open (e.g., being turned off), the stored energy is released to an output terminal 161, and the system 100 enters a demagnetization process.
Additionally, when the power switch 120 is turned off, the energy stored in the primary winding 130 is also transferred to the auxiliary winding 134 that is coupled to the primary winding 130. Consequently, the diode 108 becomes forward biased, and some energy stored in the primary winding is delivered to the capacitor 150 and used to provide a chip supply voltage 109 (e.g., VCC) to the system controller 102 through the terminal 116 (e.g., terminal VCC). The combination of the auxiliary winding 134, the diode 108, and the capacitor 150 is part of a self-supply circuit. The operating frequency of the controller 102 (e.g., the frequency of the drive signal 122) is affected by the feedback signal 158. Different output loads correspond to different magnitudes of the feedback signal 158, and thus different operating frequencies of the controller 102. For a given output load, the larger the input voltage 197, the smaller a duty cycle of the drive signal 122 becomes; and the smaller the input voltage 197, the larger the duty cycle of the drive signal 122 becomes.
FIG. 2 is a simplified diagram showing certain conventional components of the system controller 102 as part of the power conversion system 100. The system controller 102 includes a transconductance amplifier 202, a summation component 204 (e.g., an adder), an oscillator 206, a modulation component 208 (e.g., a pulse-width-modulation component), and a driving component 210 (e.g., a driver).
As shown in FIG. 2, the transconductance amplifier 202 receives the feedback signal 158 and outputs a current signal 212 (e.g., Ifb1). The summation component 204 combines the current signal 212 and another current signal 214 (e.g., I1) and outputs a combined current signal 216 (e.g., Iosc) to the oscillator 206 which generates an oscillation signal 218 (e.g., a clock signal). The modulation component 208 receives the oscillation signal 218 and the current sensing signal 126 and outputs a modulation signal 220 to the driving component 210 which generates the drive signal 122.
For example, the combined current signal 216 (e.g., Iosc) is equal in magnitude to a sum of the current signal 212 (e.g., Ifb1) and the current signal 214 (e.g., I1). For applications that need peak output power, if the combined current signal 216 (e.g., Iosc) reaches a maximum magnitude (e.g., Imax), the operating frequency reaches a maximum magnitude (e.g., Fmax).
FIG. 3 is a simplified diagram showing a conventional relationship between the operating frequency and the feedback signal 158 for the power conversion system 100. The waveform 300 represents the relationship between the operating frequency and the feedback signal 158. For example, certain magnitudes associated with the feedback signal 158 satisfy the following: FB0a≦FB1a≦FB2a≦FB3a≦FB4a. As an example, a frequency lower limit Fmin, a frequency magnitude Fnormal and a frequency upper limit Fmax satisfy the following: Fmina≦Fnormala≦Fmaxa.
As shown in FIG. 3, if the feedback signal 158 is in a range between the magnitude FB0a and the magnitude FB1a, the operating frequency is kept at the lower limit Fmina. If the feedback signal 158 is in a range between the magnitude FB1a and the magnitude FB2a, the operating frequency increases with the increasing feedback signal 158. If the feedback signal 158 is in a range between the magnitude FB2a and the magnitude FB3a, the operating frequency is kept at the magnitude Fnormala. If the feedback signal 158 is in a range between the magnitude FB3a and the magnitude FB4a, the operating frequency increases with the increasing feedback signal 158. If the feedback signal 158 is larger than the magnitude FB4a, the operating frequency is kept at the upper limit Fmaxa. If the upper limit Fmaxa corresponds to a large magnitude, a source-drain voltage (e.g., Vds) of the power switch 120 can have a large magnitude if the input voltage 197 has a large magnitude, which may lead to system damages in some circumstances.
Hence it is highly desirable to improve the techniques of peak frequency adjustment.